I attempted this project last year but I greatly miscalculated how much time my new born daughter would take! That being said, I now have some time available to dedicate to this project.
I thought of this project about six years ago while looking for a small engine that I could run off bio-fuel. To my surprise there were no light weight, compact, multi-fuel diesel engines available at a reasonable price. The idea is to develop a low cost open source two stroke diesel engine that can run off a variety of fuels. The initial design will be for a 50cc engine with the intent of being able to eventually scale to 125cc and 250cc. Such a motor could be used to power mopeds, small farm equipment, well drilling machines, pumps, generators, etc.
I have chosen the Junker Jumo engine design due to its piston configuration. On a standard diesel engine heavy cast iron cylinders and heads have to be manufactured in order to withstand the engine's compression. With the compression and fuel detonation taking place between two opposed pistons and in the center of the cylinder the walls can be thinner the further they are from the center of the cylinder. Both pistons will connect to the same crankshaft reducing the number of rotating parts. I have ran through about a dozen iterations in my head but up until now I haven't taken the time to model anything in CAD.
I have uploaded a few images of my concept. Please keep in mind that this is a quick (30 hour) and dirty (interference, exact calculations, etc. have not been taken into account) model to show the design intent. There is a lot of high level mathematical calculations and FEA (Finite Element Analysis) that will need to be done and I hope to start on those in the weeks to come.
- Simplicity (Two-Stroke)
- Variable Compression Ratio (compressor/air pump)
- Adjustable Timing (variable crankshaft)
Q and A
Q: Why a two-stroke engine?
A: Two stroke engines are less complex than four-stroke Engines, produce more power per unit volume per stroke and are less expensive to produce.
Q: Aren't two-stroke engines bad for the environment?
A: Yes, traditional two-stroke engines produce more exhaust emissions than their comparable four-stroke counterparts. This is due to the intake and exhaust ports being simultaneously open and the addition of oil to the fuel for lubrication.
Q: How will this engine be better for the environment?
A: Fuel injection and its opposed pistons design are the key here. Each piston works somewhat like a valve, by adjusting the position of the piston in the cylinder the intake and exhaust ports can be timed for efficiency. Since the fuel will be injected after both ports are closed this should result in cleaner emissions.
Q: How will it run off various fuels?
A: Compression ratio and timing need to be controlled. By using a variable crankshaft and a compressor/air pump both the timing and compression ratio can be controlled using an ECU (engine control unit).
Q: How does a variable crankshaft work?
A: The crankshaft has a series of planetary gears associated with each piston. The connecting rods are connected to a planetary gear, by rotating the ring gears the timing of the piston(s) can be accurately controlled.
Q: Are there any other benefits to using a variable crankshaft?
A: Since each connecting rod is attached to a planet gear their movement is linear, therefore eliminating cylinder side load. Side load is the most common cause of rod bearing failure and cylinder wear.
Q: What are the disadvantages of a variable crankshaft?
A: A variable crankshaft is more complex than a standard crankshaft.
I would like to clarify that I don't see these motors being manufactured on the open planes of Africa as some have assumed. I see them being manufactured in shops (home, industrial, whatever) and being used in rural areas. I want to keep the design and simple as possible so when the day comes that repairs need to be made they can be done with minimal shop availability.